1. Field of the Invention
The present invention relates to an image forming apparatus for forming a toner image on a sheet, and more particularly to an image forming apparatus including a fixing device for heating and fixing the toner image transferred onto the sheet.
2. Description of the Related Art
The conventional image forming apparatuses such as a copy machine and a laser printer include a transfer device for transferring a toner image formed on an image bearing member onto a sheet and a fixing device for heating the sheet onto which the toner image is transferred and fixing the toner image on the sheet.
For example as described in Japanese Patent Application Laid-Open Nos. 63-202776 and 4-16888, in order to stably fix the toner image onto the sheet, the conventional fixing device detects a temperature on a surface of a fixing roller that gives heat to the sheet and controls a heat amount of a heater serving as a heating device.
Various types of environments and paper are tested when the fixing device is developed, and a mutual relation between a preparation temperature on a surface of a fixing roller and a melting state of the toner on the sheet is taken into consideration to determine a preparation temperature of the heater.
However, even when a temperature on a surface of the fixing roller is controlled to be constant, the toner is differently fixed on the sheet due to a change of a surface hardness of the fixing roller or a moisture absorption state of the toner. Accordingly, the toner cannot be stably fixed on the sheet only by detecting the temperature on the surface of the fixing roller and controlling the heat amount of the heater.